Atmospheric Composition/Basic Concepts

Atmospheric Composition, Basic Concepts and Terminologies

• The atmosphere is made up of gases and vapor, and receives incoming solar energy from the sun giving rise to what we call climate.
• We live at the bottom of this indefinite layer of atmosphere where the air is densest.
• Moving up, the air thins out and the atmosphere extends up to 6000 miles above sea level.
• The lowest layer, in which the weather is confined, is known as the troposphere.Thickness of the Atmosphere
• The upper limit of the atmosphere is 10,000 km (6000 miles) above the earth surface. However, most of the meteorologists consider the top of the atmosphere at round 480 km (300 miles).

Structure of Atmosphere On the basis of compositionHomosphere and Heterosphere
• Homosphere is a zone where the constituent gases are fairly well mixed and the composition is homogeneous throughout. The Homosphere extends up to an altitude of about 50 miles above sea level.
• Heterosphere lies above the Homosphere and is a zone of poor mixing where the average composition varies depending on the location. This region starts at 50 to 60 miles (80-100 km) above the earth and, therefore, closely coincides with the ionosphere and the thermosphere.

Structure of Atmosphere on the basis of Temperature
About 50% of the atmosphere is confined to 5.6km and 97% upto 29km. On the basis of characteristic of temperature and pressure there are different layers of atmosphere.

Basic Concepts and Terminologies

ATMOSPHERE
• Atmosphere – Atmosphere is the envelope of gases surrounding the earth, held by gravity.
• Tropopause – The outer boundary of the troposphere is the Tropopause.
• Aurora : Multicoloured lights that appear in the upper atmosphere (ionosphere) over the polar regions and visible from locations in the middle and high latitudes. Caused by the interaction of solar wind with oxygen and nitrogen gas in the atmosphere. Aurora in the Northern Hemisphere are called aurora borealis and aurora australis in the Southern Hemisphere.
• Solar Wind : Mass of ionised gas emitted to space by the sun. Plays a role in the formation of auroras.

ATMOSPHERIC TEMPERATURE AND CYCLES
• Insolation : Incoming solar radiation in short wave form.
• Convection – The transmission of heat from one part of a liquid or gas to another by movement of the particles themselves.
• Conduction – Conduction takes place when two bodies of unequal temperature are in contact with one another; there is a flow of energy from the warmer to cooler body. The transfer of heat continues until both the bodies attain the same temperature or the contact is broken.
• Advection: The transfer of heat through horizontal movement of air is called advection.
• Radiation – The process by which a body emits radiant energy (energy received from the sun). It causes a loss heat, and therefore, leads to cooling
• Plank’s Law – Plank’s law states that hotter a body, the more energy it will radiate and shorter the wavelength of that radiation.
• Specific Heat – Specific heat is the energy needed to raise the temperature of one gram of substance by one Celsius
• Net Radiation – When net radiation is positive, the surface gains heat and temperature rises. When net radiation is negative, the surface loses heat and temperature falls.
• Temperature Inversion – In a temperature inversion, the normal situation of air cooling with altitude is reversed and air warms with altitude. In the stratosphere, the absorption of ultraviolet radiation causes the temperature to increase with altitude.
• Isotherms –
• Isotherms Patterns – The world patterns of isotherms are largely explained by latitude, coastal interior contrasts and elevation.
• Temperature Patterns – Six important points about temperature patterns are:
– Temperatures decrease from the equator to the poles.
– Large landmasses in the subarctic and arctic develop centers of extremely low temperatures in winter.
– Temperatures in equatorial regions change little from January to July.
– Isotherms make a large north-south shift from January to July over continents in the midlatitude and subarctic zones.
– Highlands are colder than surrounding lowlands.
– Areas of perpetual ice and snow are intensely cold.
• Temperature Range – Five important points about temperature range are:
– The annual temperature range increases with latitude.
– The greatest ranges are in the subarctic and arctic zones of Asia and North America.
– Annual range is moderately large on land in the tropical zone.
– Annual range in coastal areas is less than the range inland at the same latitude.
– Small temperature ranges are found near oceans in the tropical zone.
• Global Warming: Warming of the Earth’s average global temperature because of an increase in the concentration of greenhouse gases.
• Greenhouse Effect: The greenhouse effect causes the atmosphere to trap more heat energy at the Earth’s surface and within the atmosphere by absorbing and re-emitting long wave energy.
• Greenhouse Gases: Gases responsible for the greenhouse effect. These gases include: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); chlorofluorocarbons (CFC); and tropospheric ozone (O3). The greenhouse gases absorb long wave radiation and enhance the greenhouse effect. The burning of fossil fuels has increased the amount of CO2 in the atmosphere by 22% in the past 100 years.
• Chlorofluorocarbons (CFCs) : Is an artificially created gas that has become concentrated in the Earth’s atmosphere. This very strong greenhouse gas is released from aerosol sprays, refrigerants, and the production of fumes.
• Ozone : Tri-atomic oxygen that exists in the earth’s atmosphere as a gas. Ozone is highest in concentration in the stratosphere (10-50 km above the earth’s surface) where it absorbs the sun’s ultraviolet radiation. Stratospheric ozone is produced naturally and helps to protect life from the harmful effects of solar ultraviolet radiation.

ATMOSPHERIC MOISTURE AND PRECIPITATION
• Moisture – Water exists in the atmosphere as water vapor, clouds, fog and precipitation.
• Hydrological Cycle – The movement of water between the land, the oceans and the atmosphere is called the hydrologic cycle
• Humidity refers to the amount of water vapor in the air. The amount of water the air can hold depends on temperature. Warm air can hold more moisture than cold air.
• Relative humidity is a measure of the amount of water vapor in the air expressed as a percentage of the amount of water vapor the air can hold given its present temperature.
• Specific humidity is the actual mass of water vapor per mass of air, usually stated in grams of water vapor per kilogram of air. It is a measure of the amount of water vapor that can be extracted from the atmosphere as precipitation.
• Dew Point Temperature – The dew point temperature is the temperature at which relative humidity would be 100%. Condensation will occur if the temperature falls producing dew or frost.
• Precipitation results when a large mass of air is lifted and cooled to a temperature below its dew point.
• Adiabatic processes cause heating or cooling solely by pressure change. Air that rises expands and cools as pressure decreases with altitude. Air that descends encounters higher pressures and is compressed and warms.
• Adiabatic Lapse Rate The rate of change of temperature by an ascending or descending airmass. If no other non-adiabatic processes (i.e. no heat enters or leaves the system) occur (like condensation, evaporation and radiation), expansion causes the parcel of air to cool at a set rate of 0.98° per 100 m. The opposite occurs when a parcel of air descends in the atmosphere. The air in a descending parcel becomes compressed. Compression causes the temperature within the parcel to increase at a rate of 0.98° per 100 m.
• Dry Adiabatic Lapse Rate – A parcel of air cooling without condensation cools at the dry adiabatic lapse rate of 10° C per 1000 m (5.5° F per 1000 ft.).
• Wet Adiabatic Lapse Rate – Once air has cooled to its dew point, condensation releases latent heat, slowing the rate of cooling to the wet adiabatic lapse rate which varies between 4° and 9° C per 1000 m. (2.2° and 4.9° F per 1000 ft) depending on the temperature and pressure of the air and its moisture content.
• Condensation Nuclei – A cloud is made up of water droplets or ice formed on tiny particles of matter called condensation nuclei.
• Clouds are classified on the basis of height and form. Clouds at ground level are called fog. Radiation fog forms when the temperature of the air near the ground falls below the dew point. Advection fog occurs when warm moist air is cooled below dew point as it moves over a cold surface.
• Cirrocumulus Clouds: Patchy white high altitude cloud composed of ice crystals. Found in an altitude range from 5,000 – 18,000 m.
• Cirrostratus Clouds: High altitude sheet like clouds composed of ice crystals. These thin clouds often cover the entire sky. Found in an altitude range from 5,000 – 18,000 m.
• Cumulus Cloud: Large clouds with relatively flat bases. These are found in an altitude range from 300 – 2,000 m.
• Cumulonimbus Cloud: A well developed vertical cloud that often has top shaped like an anvil. These clouds can extend in altitude from a few hundred m above the surface to more than 12,000 m.
• Nimbostratus Clouds – Dark, gray low altitude cloud that produces continuous precipitation in the form of rain or snow. Found in an altitude range from the surface to 3,000 m.
• Precipitation – Precipitation forms when either cloud droplets or ice crystals increase in size by colliding with each other until they are heavy enough to fall.
• Hail – It is a type of precipitation received in the form of ice pellets or hail stones. The size of hailstones can be between 5 and 190 mm in diameter.
• Orographic Precipitation – Precipitation that occurs as a result of air being forced over a topographic barrier is called Orographic precipitation.
• Convectional Precipitation – Air that rises because it is warmer than the air around it produces convectional precipitation.
• Cyclonic Precipitation – Air that is forced to rise over another air mass produces cyclonic precipitation.
• Thunderstorms – Thunderstorms are intense convectional storms associated with massive cumulonimbus clouds. They may produce heavy rains, hail, thunder, lightning and intense downdrafts (micro bursts) which may create hazards for humans.

PRESSURE, WINDS AND THE GLOBAL CIRCULATION SYSTEM
• Atmospheric Pressure: Weight of the atmosphere on a surface. At sea-level, the average atmospheric pressure is 1013.25 mb. Pressure is measured by a device called a barometer. Atmospheric pressure is greatest at the earth’s surface and decreases with altitude.
• Pressure Gradients – Pressure differences between two places create pressure gradients and the resulting pressure gradient force causes air to move from high pressure areas to low pressure areas.
• Wind – Differences in pressure cause air to move horizontally. This air in motion is called wind. Winds move from areas of high pressure to areas of low pressure. Wind direction is measured by a wind vane. Wind speed is measured by an anemometer.
• Land and sea breezes are examples of winds caused by pressure differences that result from temperature differences over land and water surfaces.
• Coriolis Effect – The Coriolis Effect is due to the earth’s rotation and causes objects in motion to appear to be deflected off course. This apparent deflection is to the right in the northern hemisphere and to the left in the southern hemisphere. The effect is absent at the equator and increases as you move toward the poles.
• Frictional Force – The third force affecting the direction of wind is that of friction.
• Buys Ballot Law – Buys Ballot’s law is the relation of wind direction with the horizontal pressure distribution. According to the law, if you stand with your back to the wind in the Northern Hemisphere, air pressure will be lower to your left. If you stand with your back to the wind in the Southern Hemisphere, air pressure will be lower to your right.
• Air flow spirals into a low-pressure center and rises while the air descends and flows out of a high-pressure center. The inspiral at a low-pressure center is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. The out spiral at a high-pressure center is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
• Cyclones – (low pressure centers) are associated with cloudy or rainy weather.
• Anticyclones – (high pressure centers) are associated with clear, dry weather.
• Inter Tropical Convergence Zone (ITCZ) Zone of low atmospheric pressure and ascending air located at or near the equator. Rising air currents are due to global wind convergence and convection from thermal heating.
• Trade Winds At 30° latitude, air descends creating areas of high pressure in the subtropical high-pressure belt. Air moves out of these high pressure areas toward the equator creating the trade winds. Winds also move toward the mid latitudes creating the westerlies.
• Anabatic Winds are upslope winds driven by warmer surface temperatures on a mountain slope than the surrounding air column.
• Katabatic Wind : Any wind blowing down the slope of a mountain
• Land Breeze: Local thermal circulation pattern found at the interface between land and water. In this circulation system, surface winds blow from land to water during the night.
• Monsoons – The monsoon is a seasonally reversing wind pattern that brings heavy rains onto the Asian subcontinent in summer and hot, dry conditions in the winter.
• Geotropic Winds – Winds at an altitude of 5 to 7 km above the earth’s surface are influenced by pressure gradient force and Coriolis force but not by the force of friction. These are the Geotropic winds that flow parallel to isobars.
• Rossby waves are large undulations in the flow of the upper air Westerlies along the zone of contact between cold and warm air. They allow warm air to penetrate northward and cold air to penetrate southward.
• Jet streams are narrow bands of high velocity air that form along the polar front and above the Hadley cell in the subtropics.

WEATHER SYSTEMS
• Air Mass : A body of air whose temperature and humidity characteristics, acquired in source region, remain relatively constant over a horizontal distance of hundreds to thousands of km. Air masses develop their climatic characteristics by remaining stationary over a source region for a number of days. Air masses are classified according to their temperature and humidity characteristics.
• Front – A front is a boundary between one air mass and another. The leading edge of cold air advancing into an area is called a cold front.
• Cold Front: A transition zone in the atmosphere where an advancing cold air mass displaces a warm air mass.
• Warm Front Warm air moving into an area of cold air is called a warm front.
• Occluded Front When a cold front overtakes a warm front and forces the warm air aloft, it is called an occluded front. It’s a transition zone in the atmosphere where an advancing cold air mass sandwiches a warm air mass between another cold air mass pushing the warm air into the upper atmosphere.
• Wave Cyclone An important weather system affecting middle and high latitudes is a traveling low pressure system called a wave cyclone. Wave cyclones move from west to east and involve interaction of warm and cold fronts that often produces cyclonic precipitation.
• Tornado – A tornado is an intense low pressure system with very high wind speeds. Tornadoes develop with thunderstorms and hurricanes.
• A weather system associated with tropical areas is the easterly wave, a low pressure trough into which air converges and is lifted producing precipitation.
• A polar outbreak occurs when cold polar air forces its way into very low latitudes, bringing storms followed by cold, clear weather.
• Tropical cyclones, hurricanes and typhoons are all names for powerful storms which develop over warm ocean surfaces between 8° and 15° latitude, migrate westward and curve toward the poles. Tropical cyclones often create tremendous damage due to high winds, high waves, flooding and heavy rains.